1. Field of the Invention
This invention relates to metal detectors operating according to the pulse induction principle which have the ability to sense metal objects deeply buried within conductive salts and heavily mineralized ferromagnetic sands.
2. Description of the Prior Art
Pulse Induction metal detection devices are well known in the art and are highly regarded for their ability to operate in extreme ground conditions. An example of such a device is shown in Colani, U.S. Pat. No. 3,315,155. Such devices utilize a pulse generator connected to a power amplifier. The power amplifier sends a short pulse of high current into a search coil, which consists of a simple coil of wire, producing a magnetic field. As the current is cut off, the collapsing magnetic field generates a "reflected pulse" which is opposite in polarity and many times greater in amplitude than the original current pulse. The reflected pulse causes electric currents to flow in any metal objects within its range. These electric currents act upon the reflected pulse so as to lengthen the time it takes to decay to zero. The increase in time changes the slope of the decaying portion of the reflected pulse. This change in the slope is detected and converted to a DC potential which is proportional to the change in the duration of the reflected pulse, and controls the audio indicating device to alert the user to the presence of a buried metal object.
Most pulse induction metal detectors previously known in the prior art operate at a pulse transmit frequency in the range of 85 to 150 pulses per second. The duration of the transmit pulse is typically in the range of 365 microseconds.
In practice, the Pulse Induction metal detectors known in the prior art have several disadvantages. In order for such devices to be able to detect a metal object at some distance from the search coil, they must have a very high gain receiver. A high gain receiver, however, which increases sensitivity, also results in an increase in the amount of random noise and false signals.
The overall sensitivity of the Pulse Induction metal detector depends upon many factors, the most important being the point at which the sampling occurs. The sampling point in most pulse induction metal detectors generally falls between 40 and 80 microseconds after the leading edge of the reflected pulse as measured at the output of the high gain amplifier. Reduced delay times can increase the sensitivity, but also increase noise, drift and instability. Adjusting the tuning controls can be especially difficult due to the DC coupling between the amplifier stages. Increasing the delay times improves stability and reduces noise, but causes a corresponding reduction in the depth of detection. Lower conductivity metals, such as gold, platinum and cupro-nickel alloys, may not be detected at longer sampling delays.
Pulse samples must be integrated over time and pulse induction metal detectors known in the art use a relatively long integration time constant, typically between 220 and 1000 msec. The long time constant averages the sampled pulses over a longer time to reduce noise and improve the signal to noise ratio and stability of the DC coupled circuits. In practice, these long integration time constants require that a slow coil sweep be used. If the coil is swept too quickly, the integrator may not respond to a metal target and there will be no audio response. Long integration time constants also produce a broad audio signal which causes some difficulty in locating the exact center of the target.
Pulse induction metal detectors commonly use a VCO audio circuit that can be very critical to adjust. A DC offset control is set so that the VCO stage remains on the verge of producing an audio signal. This is usually the point where an occasional clicking or growling sound is heard. This sound changes to a high-pitched squeal when a metal object is in the field of the search coil. The pitch increases in proportion to the reduction in the distance between the metal object and the search coil, where the pitch of the audio tone is used to locate the center of the object being detected. In practice, the VCO audio can be erratic, unstable and annoying. The operator must check the offset adjustment frequently to be sure the VCO stage is set properly or a reduction in the depth of detection will result.